Centrifugal compressor

ABSTRACT

A centrifugal compressor is shown, comprising: an impeller; a rotatably mounted diffuser surrounding the impeller; and a driving arrangement configured to drive the impeller and diffuser to rotate in opposing directions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority fromUnited Kingdom patent application number GB 2113165.1 filed on Sep. 15,2021, the entire contents of which is incorporated herein by reference.

BACKGROUND Technical Field

This disclosure relates to a centrifugal compressor.

Description of the Related Art

Compressors suitable for compressing low density gases such as heliumand hydrogen are typically centrifugal, single-stage, and usually offerlow compression ratios. To achieve higher pressure ratios, compressorsmay be linked together with multiple individual compressors in series.This increases weight and cost, as well as increasing the likelihood oflosses. Existing material limits for compressors are generally known andfixed. A titanium impeller for example is limited to its outer edgeperiphery operating at a maximum speed of around 550 metres per second.Since the speed of sound of helium at atmospheric conditions is over1000 metres per second, compression is difficult to achieve in a singlestage design. Helium in particular suffers from a high ratio of specificheats, meaning that more heat is generated through compression thannormal fluids as a result of the molecular degrees of freedom available(three) compared to diatomic molecules (five or six).

SUMMARY

In an aspect there is provided a centrifugal compressor comprising:

an impeller;

a rotatably mounted diffuser surrounding the impeller; and

a driving arrangement configured to drive the impeller and diffuser torotate in opposing directions.

In an embodiment, the driving arrangement may comprise a motor and agearbox. The gearbox may be an epicyclic gearbox comprising a sun gear,a planetary gear and a ring gear, the impeller being connected to thesun gear and the diffuser connected to the ring gear.

In an embodiment, a first end of a rotor of the motor may be connectedto the sun gear and a stator of the motor fixed relative to theplanetary gear.

In an embodiment, where the impeller is a first impeller, the diffuseris a first diffuser and the epicyclic gearbox a first epicyclic gearbox,the compressor may further comprise:

a second epicyclic gearbox comprising a sun gear, a planetary gear and aring gear;

a second impeller connected to the sun gear of the second epicyclicgearbox and to a second opposing end of the rotor; and

a second diffuser connected to the ring gear of the second epicyclicgearbox.

In an embodiment, the driving arrangement may comprise first and secondmotors, the impeller and diffuser being connected to respective rotorsof the first and second motors.

In an embodiment, the impeller may comprise vanes angled towards adirection of rotation of the impeller. The diffuser may also comprisevanes angled towards a direction of rotation of the diffuser.

In an embodiment, the centrifugal compressor may further comprise aninlet inducer in an inlet gas path of the compressor.

In an embodiment, the centrifugal compressor may further comprise ahousing surrounding the diffuser and a seal between the housing and thediffuser. The seal may be a labyrinth seal.

In an embodiment, the centrifugal compressor may be used for thecompression of gas. The centrifugal compressor may be used for thecompression of neon, or hydrogen, or helium.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the accompanying drawings, which are purely schematic and not toscale, and in which:

FIG. 1 is a schematic diagram illustrating example impeller designs withrearward-facing, perpendicular and forward-facing impeller vanes;

FIG. 2 is a schematic sectional diagram of an impeller and diffuserarrangement of an example centrifugal compressor; and

FIG. 3 is a schematic sectional diagram of an example centrifugalcompressor.

DETAILED DESCRIPTION

FIG. 1 illustrates three different example impeller designs for acentrifugal compressor. In each case the impeller 101 a-c has a centralhub 102 a-c and a plurality of radially extending vanes 103 a-c. In afirst example, the impeller 101 a has backward-swept vanes 103 a, inwhich the vanes 103 a extend radially from the hub 102 a angling againstthe direction of rotation R, with an angle β between the vanes 103 a anda tangent of an outer circumference 104 being less than 90 degrees. In asecond example, the impeller 101 b has radial vanes 103 b that remain inline with a radial direction from the hub 102 b, with each vane 103 bbeing perpendicular to the outer circumference 104 such that the angle βis 90 degrees. In a third example, the impeller 101 c has forward-sweptvanes 103 c, in which the vanes 103 c extend radially from the hub 102 cangling towards the direction of rotation R, with the angle β beinggreater than 90 degrees. The corresponding velocity triangles below eachexample indicate the impeller gas exit velocity v₂, which increases asthe blade angle β increases. Forward-swept vanes 103 c therefore allowfor a more rapid acceleration of gas flow and consequently a more rapidpressure increase. This results in an increased compression ratio,allowing for more efficient compression in a single stage. In exampleembodiments having forward-swept vanes, the angle β may be betweenaround 100 and 170 degrees.

An end view of an example centrifugal compressor 200 is illustrated inFIG. 2 . The compressor 200 comprises an impeller 201 and a diffuser202. The impeller 201 comprises radially outwardly extending vanes 203that are angled towards a direction of rotation R_(I) of the impeller201, i.e. forward-swept. The diffuser 202 comprises radially inwardlyextending vanes 205 that are angled towards a direction of rotationR_(D) of the diffuser 202, i.e. also forward-swept. In conventionalapplications, the diffuser 202 will be fixed and the impeller rotatable,the compressor 200 comprising a motor configured to drive the impeller201. In the present embodiment, however, both the impeller 201 anddiffuser 202 are rotatable and the compressor 200 is configured torotate the impeller 201 and diffuser 202 in opposing directions. Anadvantage of this arrangement is that there is a higher relative tipspeed between the impeller and diffuser. With the above-mentionedmaterial limits providing an upper limit on the tip speed of theimpeller vanes, the higher relative speed allows for a relative tipspeed to be up to Mach 1 even for low density gases such as helium,compared to traditional impellers where the tip speed may be limited toMach 0.5 or lower. Using such a contra-rotating impeller-diffuserarrangement, the relative speed between the impeller 201 and diffuser202 can effectively be doubled, allowing not only more efficientcompression in a single stage, but also allowing existing materials tobe used.

FIG. 3 illustrates an example centrifugal compressor 300 comprising acontra-rotating impeller and diffuser arrangement. As indicated in FIG.3 , static parts are non-patterned whilst moving components arepatterned. The compressor 300 comprises a first impeller 201 a and afirst diffuser 202 a. Gas G to be compressed enters the compressor 300through a gas path 302, which passes through the impeller 201 a anddiffuser 202 a and exits the compressor 300 from an outer circumferenceof the diffuser 202 a.

A driving arrangement is configured to drive the impeller 201 a anddiffuser 202 a in opposing directions. The driving arrangement comprisesa motor 301 and a gearbox 303. The motor 301 drives the impeller 201 aand diffuser 202 a in opposing directions via the gearbox 303. In thisexample the gearbox 303 is an epicyclic gearbox comprising a sun gear303 a, a plurality of planetary gears 303 b and a ring gear 303 c. Thesun gear 303 a is driven by the rotor 301 a of the motor 301. Inalternative arrangements the motor 301 may drive the ring gear 303 cinstead. The planetary gears 303 b are fixed relative to each other andto the stator 301 b of the motor 301, which causes the outer ring gear303 c to rotate in an opposing direction to the sun gear 303 a. A ratiobetween the rotational speeds of the ring gear 303 c and sun gear 303 ais selectable by selecting the relative sizes of the gears 303 a-c. Therotor 301 a and sun gear 303 a are connected to the impeller 201 a,while the ring gear 303 c is connected to the diffuser 202 a. The ringgear 303 c may be integral with the diffuser 202 a or may be separatecomponents that are joined to each other.

A second impeller 201 b and a second diffuser 202 b is also be driven bythe same motor 301 via a second gearbox 304. In this way, axial loadsmay be balanced. The second gearbox 304 is similar to the first gearbox303 in having a sun gear 304 a connected to the impeller 201 b and to anopposing end of the rotor 301 a, planetary gears 304 b fixed relative tothe stator 301 b and an outer ring gear 304 c connected to the diffuser202 b.

Alternative examples which are outside the scope of the presentapplication may include separate motors driving the impeller 201 a anddiffuser 202 a. An advantage of using a single motor is that thecompressor may be made more compact.

The compressor 300 comprises a housing 305 surrounding the motor 301 anddiffuser 202 a. The housing may also define the gas path 302. A seal isrequired between the housing 305 and the diffuser 202 a that preventsgas from leaking away from the gas path 302. The seal may be in the formof a labyrinth seal 306 used between the ring gear 303 c and the housing305. Other ways of sealing against the housing 305 may alternatively beused, such as an air bearing seal.

In some examples, an inlet inducer may be provided in the gas path 302leading to the impeller 201 a to increase a static pressure of gasentering the compressor 300.

Using a single motor 301 to drive impellers 201 a, 201 b and diffusers202 a, 202 b at opposing ends of the rotor 301 a has a further advantageof allowing high aerodynamic load stresses to be better managed as wellas providing improved gas sealing, which is particularly difficult forlow density gases such as helium. In alternative examples a single-sidedcompressor may be sufficient.

A centrifugal compressor 200 of the type disclosed herein may beparticularly useful for compressing gases that are normally moredifficult to compress such as neon, hydrogen, and helium. A compressorof the type disclosed herein may also enable smaller and lightercompressors for other gases, particularly where fixed pressure ratiosare desired.

Various examples have been described, each of which comprise variouscombinations of features. It will be appreciated by those skilled in theart that, except where clearly mutually exclusive, any of the featuresmay be employed separately or in combination with any other features andthus the disclosed subject-matter extends to and includes all suchcombinations and sub-combinations of the or more features describedherein.

1. A centrifugal compressor comprising: a first impeller; a firstrotatably mounted diffuser surrounding the impeller; and a drivingarrangement configured to drive the first impeller and first diffuser torotate in opposing directions, the driving arrangement comprising amotor and a first epicyclic gearbox comprising a sun gear, a planetarygear and a ring gear, the first impeller being connected to the sun gearand the first diffuser being connected to the ring gear, the compressorfurther comprising: a second epicyclic gearbox comprising a sun gear, aplanetary gear and a ring gear; a second impeller connected to the sungear of the second epicyclic gearbox and to a second opposing end of therotor; and a second diffuser connected to the ring gear of the secondepicyclic gearbox; wherein a first end of a rotor of the motor isconnected to the sun gear of the first epicyclic gearbox and a secondend of the rotor of the motor is connected to the sun gear of the secondepicyclic gearbox and a stator of the motor is fixed relative to theplanetary gear.
 2. The centrifugal compressor of claim 1, wherein thefirst and second impeller each comprises vanes angled towards adirection of rotation of the respective impeller.
 3. The centrifugalcompressor of claim 1, wherein the first and second diffuser eachcomprises vanes angled towards a direction of rotation of the respectivediffuser.
 4. The centrifugal compressor of claim 1, further comprisingan inlet inducer in an inlet gas path of the compressor.
 5. Thecentrifugal compressor of claim 1 further comprising a housingsurrounding the first and second diffuser and a seal between the housingand the diffusers.
 6. The centrifugal compressor of claim 5, wherein theseal is a labyrinth seal.
 7. A method of compressing a gas, the methodcomprising: rotating a first impeller with a first sun gear of a firstepicyclic gearbox, the first sun gear driven by a first end of a rotorof a motor; rotating a first diffuser surrounding the first impeller inan opposite direction to the first impeller using a first ring geardriven by a first planetary gear in the first epicyclic gearbox; passinggas through the first impeller and the first diffuser.
 8. The method ofclaim 7, wherein the gas is neon, hydrogen, or helium.
 9. The method ofclaim 7, wherein the method further comprises: rotating a secondimpeller with a second sun gear of a second epicyclic gearbox, thesecond sun gear driven by a second end of the rotor of the motor;rotating a second diffuser surrounding the second impeller in anopposite direction to the second impeller using a second ring geardriven by a second planetary gear in the second epicyclic gearbox; andpassing gas through the second impeller and the second diffuser.
 10. Themethod of claim 9, wherein the gas is neon, hydrogen, or helium.